DUAL CPU board vs 2 Single CPU boards: bang for buck?

[Robert G. Brown]

On Thu, 7 Mar 2002, Jim Lux wrote:

> >Looking at the pictures in the cited website.. Since Beowulfs have often 
> >been built as commodity boxes on bakery rack shelving.. Why not take it a 
> >step further and just mount the stuff flat on a cookie sheet or eclair 
> >pan, high density racks for which are readily available.
> 
> There is significant advantage in actually having a real chassis (mounting 
> holes, etc.), but, for the low budget approach, "gang drilling" a bunch of 
> cookie sheets seems a real possibility, and/or, the strategic use of double 
> stick foam tape, glue, or hook/loop fasteners(Velcro).
> 
> Sure, if you glue the boards/diskdrives/etc down to the sheet it makes 
> servicing hard, but on the other hand, these things are almost cheap enough 
> that you could just throw it away if it breaks.. (I could make a good case 
> for gluing down the power supply... $15-20 power supply and $2 cookie sheet....

Don't forget to add the cost of your time...

Also, don't underestimate the value of the engineering in a rackmount
case.  Yes, they're big and heavy.  And they're strong, as they have to
be to go in a rack and cantilever-support their entire weight by screws
on the front face.  And they're easy to insert and remove for
maintenance.  And they cool their contents very predictably, with fans
and holes in (one hopes:-) the right places.  And they come with risers
that let you add 1-3 cards in less than the height of a PCI card.  And
they support the wiring and motherboard in a configuration reasonably
likely to NOT short out if you jar it a bit or bump the rack from the
side.

Cookie sheets don't.  In fact, a "good" cookie sheet (one with good
structural properties) costs more than $2 too -- even in cookiemaking,
you get what you pay for.

But time, now that is a real cost and needs to be counted in a CBA.

My favorite time/cost estimate runs like this:

  "Hmm, one hour of My Time or a hundred dollars of Other People's
Money?"

Goodness. Decisions, decisions...;-)

Of course, if it is YOUR money and opportunity cost time (as it is for a
home system or a hobby) then by all means, DIY.  As I said, I build my
own systems for my >>home<< beowulf.  Maybe I'll even try the cookie
sheet design one day...;-)

But for a "large" cluster?  Yerrch.  I'm an odd sort of person.  I mean,
I sort of like making cat 5 cables myself -- it is a soothing thing,
kind of like knitting.  Even so, the thought of assembling, powering up,
testing, debugging, fixing, shelf mounting 64 or so cookie sheet nodes
does NOT tremendously appeal.  Wires, fans, hot melt glue, SPACERS
(don't want to short out that motherboard) and when you are done the
whole thing will probably radiate like all hell out the sides and screw
up everything from mobile phones to FM radio in the near vicinity.

(Don't forget that "real" cases are FCC approved to trap emissions that
can interfere with all sorts of equipment -- I remember well my first
IBM PC (64K motherboard) back in '82.  When it was on I couldn't listen
to FM radio in a fairly wide band of frequencies in the same room, as
the early cases did NOT shield adequately.  Lots of electrons bouncing
along lots of paths.)

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu